In modern firearms, the cartridge comprises both the gunpowder and the primer and bullet in a single firing module.
The cartridge consists of a cylindrical housing, referred to as the cartridge case, generally made of brass, containing the gunpowder. Said cartridge case is closed at the two ends by the bottom and by the actual bullet itself. The primer for the powder is contained within a capsule, referred to as the percussion cap, located in the bottom.
The impact of the percussion cap causes the priming mixture to detonate giving rise to an intense burst of flame and then to the deflagration of the gunpowder. The combustion of the powder then propagates to the entire mass of the latter in a very short time. While burning, the powder generates a large amount of high temperature gas, the pressure of which, as the gases are confined within a small volume, rises rapidly to very high values. The gas expansion, contained by the walls of the cartridge case, contained in turn by the walls of the combustion chamber, and by the bottom which thus remains pressed against the bolt, may occur only in the axial direction against the bottom of the bullet. The bullet is thus accelerated, pushed by the gases, the pressure of which will continue to increase until the bullet has left the cartridge case thus beginning its travel within the firearm barrel. More precisely, the gas pressure increases until the increase in gas volume will exceed the available space, i.e., until a balance has been achieved between the increase of volume and the available space, which balance is generally achieved once the bullet has traveled a few cm. At this point, the pressure will have reached its peak value and will then begin to decrease as the available space between the bottom and the base of the bullet increases. The pressure drop is also due to the fact that a part of the gas energy is converted into kinetic energy of the bullet and a part is also used to overcome the friction along the barrel which is very high for rifled barrels. Therefore, the bullet is pushed at ever-increasing speeds until it leaves the firearm barrel.
Gunpowders may roughly be divided into slow burning or fast burning in relation to the combustion process. Since the so-called fast-burning powders deflagrate faster, they produce a generally narrower and higher peak pressure than the so-called slow-burning powders. The 12.7×99 mm caliber NATO cartridge, otherwise known as .50 BMG, for example, uses powders having a particularly “slow” combustion rate, a pressure peak of about 4000 BARS and a muzzle velocity of about 900 m/s for a barrel of about one meter. As the bullet exits, the gas pressure will have reached a value of about one-fifth of its peak value. It is worth controlling the combustion so that it remains confined within the barrel, i.e., the powder must burn completely before the bullet has left the barrel, both to exploit all the potentially available energy, and to prevent the residuals from igniting beyond the muzzle of the firearm. Moreover, the peak and muzzle pressures are required to not exceed certain values and their ratio needs to remain within the limits mentioned above.
Assuming the use of a propellant burning more quickly than that described, i.e., a propellant characterized by a higher combustion rate, in the first firing situation, an excessive rise of peak pressure could occur, being large enough to abundantly exceed the strength limitations of the mechanical barrel/bolt assembly. Therefore, in extreme cases, a bursting effect could occur instead of a propulsive effect. Furthermore, an excessive peak value could lead to a premature wear of the barrel rifling. It is thus important to find new ignition devices which regulate the pressure values so that the combustion can take place in an increasingly safe and efficient manner even if high-combustion-rate propellants are used.